The need for lower cost electricity produced with reduced adverse environmental impacts has created a great deal of interest in fuel cells which create electricity by chemical reactions at electrodes. The outstanding advantage of the fuel cell is the very high efficiency by which it can convert the thermodynamic energy potential of the reactants into electricity. This efficiency can be as much as twice the efficiency of thermal conversion methods such as steam turbines and internal combustion engines. Additionally, the fuel cell is a mechanically simple device, lending itself to compact and comparatively inexpensive installations. Further, as the process does not involve extreme temperatures or large gas flows for the energy-producing source, there are excellent opportunities to insure the recovery of undesirable impurities. A great deal of current fuel cell development is being placed on hydrogen fuel cells with their advantageous oxidation product of water.
Hydrogen, despite the ease of its use and attractive water by-product, has certain disadvantages. For example, hydrogen is very difficult to store. Because it can be liquified only at extremely low temperatures, it is practically stored at very high pressures in cylinders of great strength, or stored as a compound such as metal hydrides, or in nano-sized carbon tubes. In all of these alternatives, the light weight hydrogen is less than 15% of the weight of the storage device.
The production of hydrogen of a purity suitable to sustain fuel cell use is another difficulty with the current fuel cell technology. Electrolytic production, while meeting purity goals, has heretofore presented no electric energy advantage. Production by the reforming of natural gas (primarily methane) requires a large energy input for the reforming reaction, and starts from an increasingly expensive material. Producing hydrogen by the water gas reaction suffers from inherent difficulties including the production of carbon monoxide and an endothermic reaction to which large amounts of heat must be supplied, making it a complex and expensive process. Further, the carbon monoxide produced in the reaction is a poison to hydrogen cells, requiring difficult carbon monoxide reactions or separations to achieve suitable hydrogen quality from this process.
When compared to hydrogen, carbon is widely available. Concentrated in coal, it is the preferred and most heavily-used source of energy in the world for the production of electricity. Carbon-containing organic materials are ubiquitous in nature in forms such as wood, paper, plastics, cloth, and rubber. These materials constitute the major components of land-filled waste. All of these materials may be efficiently converted to carbon and water as described in international application PCT/US2004/012343 (WO 2004/096456 A2) which is incorporated herein by this reference. Thus, carbon is available both inexpensively and with environmental advantage as a source for electricity production.
It has long been recognized that it would be very advantageous if carbon could be electrolytically-processed to either hydrogen, or directly to electricity. U.S. Pat. No. 4,226,683 to Vaseen describes an electrolytic cell that converts carbon to hydrogen by the carbon-water reaction. The oxygen in the water producing carbon dioxide at one electrode, while hydrogen is produced at the second electrode. The cell operates at a high temperature (180° C.) and requires a high pressure containment to overcome water's gas state at this temperature. The cell further requires a circulating organic depolarizer to remove the carbon dioxide and hydrogen from the system.
U.S. Pat. No. 6,200,697 to Pesavente describes a carbon-air fuel cell. The cell operates at 400° C. in mixed fused metal hydroxides. Water is introduced as a gas in the incoming air (oxygen) stream. The reaction of water with certain chemicals assists in the discharge of carbon dioxide from the carbonates formed in the reaction. The high temperature involved and the complexity of the carbon dioxide discharge are disadvantages of this system.
Cherepy et al. (Journal of the Electrochemical Society, 152(1):A80 January, 2005) demonstrate a carbonate fused salt fuel cell, without hydrogen ion, operating at 800° C. which has a woven ceramic separator. This cell combines the reaction of carbon and carbonate ions at the anode to produce electrons and carbon dioxide, with oxygen oxidation of carbon dioxide with electrons at the cathode to produce carbonate ions. However, the high temperature operation of this cell concept is particularly disadvantageous.
Therefore, there is a long felt need for a means of producing energy in a fuel cell using carbon as a fuel source, which operates at a practical temperature. Preferably, the system would also generate high purity hydrogen at a commercially-acceptable price.